1. Field of the Invention
This invention relates to sampled data systems and, more specifically, to a charge neutralizing system for switched current systems having charge injection problems and method therefor.
2. Description of the Prior Art
Charge injection has been a major problem for switched current systems. In a switched current system, a signal is sampled by a transistor switch. The charge injection problem occurs when the transistor switch is turned off. The charge stored in the transistor switch when the transistor switch is activated has to be discharged to disconnect the transistor switch. For an NMOS transistor switch, the negative channel charge is induced when the NMOS transistor switch is activated. However, when the NMOS transistor switch is deactivated, the negative channel charge must be neutralized by a positive charge. A positive charge stored in a parasitic capacitor or a load capacitor of the drain or source of the NMOS transistor switch must be supplied in order to neutralize the negative channel charge. The neutralization of the negative channel charge by the positive charge lowers the charge stored in the drain and source nodes of the transistor switch which causes the charge injection problem.
In order to reduce the charge injection problem, a dummy transistor or a replica circuit technique have been proposed. While these two proposals do work, they produce several problems. The dummy transistor technique uses two clock phases. This in turns makes the dummy transistor technique very difficult to control since the accuracy is critically dependent on the clock phase edges. The replica circuit however, while not dependent on clock phase edges, consumes a larger amount of area. The larger use of area and power consumption makes the replica circuit technique uneconomical to use.
Therefore, a need existed to provide an improved system and method for reducing charge injection problems in a transistor switching circuit. The improved system and method will supply a charge to neutralize the channel charge stored in the transistor switch when the transistor switch is disconnected. The improved system and method will not be critically dependent on clock phase edges. The improved system and method will also not consume large amounts of power or large areas of silicon real estate thus making the system economically practical to install.